The invention relates to a process for producing a cast metal strip using two casting rolls and two side plates, which together form a melt space and a casting gap, metal melt being fed into the melt space and in the melt space forming a melt bath with a bath surface which is open at the top, and a cast metal strip being delivered out of the melt space through the casting gap, and a delimited surface region for the collection of particles which are foreign to the melt being formed on the bath surface under the action of at least one gas jet, and to a two-roll casting device used for this process.
The invention preferably relates to a casting process for producing a continuously cast steel strip with a strip thickness of between 0.5 mm and 10 mm using a two-roll casting installation, with the cast steel strip being removed substantially vertically downward.
A two-roll casting device with a vertically delivered metal strip is generally known and comprises, as is diagrammatically illustrated in FIGS. 1 and 2, two driven, oppositely rotating casting rolls 1, 2 and two sides plates 3, 4, which are preferably placed against the end sides of the casting rolls and thereby form a melt space 5 for receiving metal melt introduced through a submerged casting nozzle 6. The two axes of rotation of the casting rolls lie in a horizontal plane and are arranged parallel to and at a distance from one another, so that a casting gap 7 is formed between the casting rolls; the longitudinal extent of this casting gap 7 is delimited by the side plates, and therefore the casting gap 7 has a cross section which corresponds to the cross section of the desired cast strip. With continuous supply of metal melt into the melt space, a melt bath with a bath surface 8 that is open at the top is formed therein. Above the bath surface, the melt space is delimited by a covering hood 9, which bears, either so as to form a seal or leaving clear a gap, against the casting rolls and side plates, in order to substantially prevent the access of external air. At the bottom, the melt space opens out into the casting gap, from which the metal strip emerges. When the casting rolls are rotating, starting from the contact lines 10, 11 between the bath surface and the cooled casting rolls, two strand shells 12 are formed on the lateral surfaces of the casting rolls where they enter the melt bath, the strand shells becoming continuously thicker and ultimately being combined in the casting gap to form the metal strip 13.
With a continuous supply of metal melt into the melt bath through the submerged casting nozzle, which causes movement in the melt bath, nonmetallic particles which are foreign to the melt are entrained. These particles float to the surface of the bath, where they agglomerate, together with particles which are foreign to the melt and were generated in the mold melt bath by chemical reaction with refractory material or by reoxidation, and are incorporated in the strand shells predominantly at the contact line with the casting rolls directly at the lateral surface of the casting rolls, forming inclusions and seeds for macrocracks and microcracks at the surface and in the region close to the surface of the cast metal strip.
A two-roll casting installation and a casting process for casting a metal melt in accordance with the prior art described is known, for example, from JP-A 2001-314946, WO 02/083343 and JP-A 2-207946.
To keep particles which are foreign to the melt away from the contact line between the casting-roll surface and the bath level surface, it is proposed in JP-A 2001-314946 that gas jets be applied in the region of this contact line, causing the particles which are foreign to the melt to drift away toward the center of the melt pool. The gas jets cover part of the casting roll surface and an edge region of the bath level surface, but bath fluctuations and temperature fluctuations which influence the strand shell growth occur at the casting roll surface in a sensitive area depending on the intensity and temperature of the gas jets. Unfortunately, substantially uniform starting conditions for the formation of the strand shells in this region are particularly important for the end product.
According to WO 02/083343, drifting of particles which are foreign to the melt and have been entrained into the melt bath toward the contact line between the metal bath and the lateral surfaces of the casting rolls is avoided during casting operation by means of shields which are obliquely immersed in the metal bath and the lower edges of which are positioned below the level of the outlet openings of the submerged casting nozzle. The intention of this is to additionally create a melt pool in the melt space, in which the nonmetallic particles can be separated off. The metal strip which is produced continuously using the two-roll casting device is wound into coils, and at the end of the winding operation of each individual coil, the shields are removed from the metal bath and the particles which have been separated out at the surface of the bath are blown toward at least one of the casting roll surfaces using gas nozzles and in this way discharged together with a short piece of the metal strip. The main drawback of this process is that each cast coil produces a piece of scrap, which interrupts the continuous production process and increases the scrap rate of production. Furthermore, metal melt accumulates on the shields and solidifies each time the shield is raised. If the shield consists of refractory material, eroded particles of the refractory material are additionally introduced into the melt, or chemical reactions occur between the liquid steel and the refractory material, which produce additional impurities.
JP-A 2-207946 has disclosed a two-roll casting device in which the foreign particles floating on the bath surface are removed by being continuously scooped out using rotating cup mechanisms. Since these devices at the bath surface have to work at the melting point of the metal, there is likely to be a high number of operating faults in these mechanical devices. In addition, in the case of a steel bath, the bath surface has to be protected from contact with atmospheric oxygen, and consequently it is not feasible to use scoop devices of this type under these conditions.